This invention relates to three-antenna methods for measuring antenna polarization. Polarization measurements to determine tilt angles, axial ratios and sense of electric field polarization ellipses of antennas by means of procedures carried out on antenna measurement ranges are well known. In general, these parameters are functions of direction, and a polarization measurement of the field of an antenna must therefore be made at each pattern direction of interest. Such solid angle antenna polarization measurements require knowledge of the polarization of the range antenna with which the measurement is made.
When the range antenna itself has unknown polarization characteristics, a three-antenna polarization measurement is performed to determine the polarization characteristics of the range antenna. Because of the complexity of the data reduction associated with a three-antenna polarization measurement procedure, such a procedure is usually limited to determining the polarization of an antenna in only one direction of radiation pattern. By performing polarization pattern measurements on each of the three possible pairs of antennas in a three-antenna set, one is able to ascertain the axial ratio, tilt angle and sense of the polarization ellipse for each of the three antennas. Typically, the polarization for the direction of the peak of the beam for each of the three antennas is measured. The only apriori knowledge required is the approximate tilt angle (within 45 degrees) of one of the antennas in the three-antenna set and the fact that one of the antennas is reciprocal.
According to a previously known three-antenna measurement technique, a first pair of antennas is positioned on a test range at a sufficient distance from one another to satisfy far-field conditions and so that they share a common boresight or roll axis. The receiving antenna is rotated in a selected direction about its roll axis until a minimum transmission voltage is located, and then rotated 90.degree. in the opposite direction. In this orientation, the amplitude is set to one, the phase to 0.degree. and the rotation angle from the roll angle reference is recorded. The receiving antenna is again rotated to the minimum location, where the direction of phase change during rotation and the amplitude at the minimum are recorded. This process is repeated for the other two pairs of antennas and polarization ratios are calculated from this data. The tilt angle for each antenna is determined from the measured rotation angles. Aspects of such techniques are described in Newell, A. C., "Improved Polarization Measurement Using a Modified Three-Antenna Technique", Proceedings of the IEEE International Antennas and Propagation Symposium, Urbana-Champaign, Ill., June 2-4, 1975, Session 15, pp. 337-340, 1975. Other three antenna polarization measurement techniques, less accurate in general than that referred to above, are described in Newell, A. C., and Kerns, D. M., "Determination of Both Polarization and Power Gain of Antennas by a Generalized 3-Antenna Measurement Method", Electronic Letters, Vol. 7, No. 3, pp. 68-70, Feb. 11, 1971; Joy, E. B., and Paris, D. T., "A Practical Method for Measuring the Complex Polarization Ratio of Arbitrary Antennas", IEEE Transactions on Antenna and Propagation, Vol. AP-21, No. 4, pp. 432-435, July, 1973 ; and Newell, A. C., Baird, R. F., and Wacker, P. F., "Accurate Measurement of Antenna Gain and Polarization at Reduced Distances by an Extrapolation Technique", IEEE Transactions on Antennas and Propagation, Vol. AP-21, No. 4, pp. 418-431, July, 1973. All of those publications are incorporated herein by this reference.
Certain inaccuracies are inherent in such measurement techniques, however. First, the polarization ratios for the antennas in measurements such as those described in the references above are computed from only two measured received phasor voltages for two relative roll angles of the antennas. The measured data for these two orientations is subject to corruption by the presence of ambient and reflected energy in the test range, by variations in the transmitter and receiver radio-frequency (RF) paths, and by random instrumentation errors.
Second, because the transmitting antenna is fixed, rather than being rotated, the measurement procedures described in the references above all fail to account for ambient and reflected energy in the test range and for variations in the RF path connecting the rotating antenna to the fixed portions of the measurement system.
Third, each of the methods of the references described above, except for the first, require extremely accurate measurements of relative phase between the two measurement orientations of each pair of antennas. In some unusual measurement situations involving these methods, all of the polarization information in the measurement may be contained in the phase measurement. Measurement of relative phase is inherently more difficult and less accurate than measurement of relative amplitude, so that all of the other methods described in the references above are inherently less accurate than that described in the first reference.
Finally, the method described in the first reference requires use of three coordinate systems, one fixed by reference to each antenna and one fixed in space by reference to the test range. Coordinate system redefinition is a requirement when using this process, and this redefinition may lead to confusion and errors.
A three-antenna antenna polarization measurement method in which one antenna in each pair of antennas is rotated about the common axis, the phase and amplitude of the received signal is sampled for every degree in a 0.degree.-360.degree. rotation and such data is converted in a minicomputer via Fast Fourier Transform ("FFT") is disclosed in E. L. Christensen, et al., Experimental Near Field Antenna Test Facility Phase 2 Final Report (European Space Agency Contract Report, Contract No. 3916/79/NL/DG) pp. 93-99 (vol. 1 December, 1981) (the "Technical University of Denmark Report"), which is incorporated herein by this reference. This report describes work carried out by the Electromagnetics Institute at the Technical University of Denmark.